Ultrasonic images made in a media that is inhomogeneous in the acoustic refractive index, i.e. in its speed of sound, generally exhibit focusing errors. For imaging systems utilizing multi-element transducer arrays and adjustable time delay means connected to operative elements of the array for focusing and beam steering, it is possible to correct such focus errors to some degree by imposing adjustments to the time delays.
One prior art method for accomplishing reduction of focus defects includes cross correlating short segments of the signals received by adjacent or nearby array elements, corresponding to a point in the object space for which the correction is to be made. The cross-correlation coefficient should be maximum at a time shift equal to the beam steering and focusing delay required to image at that point. If it differs, a time shift correction is introduced to bring it into conformity. All time-shifted corrected waves are summed. Cross-correlation methods for correcting for distorting medium are shown in U.S. Pat. No. 4,484,477 by Buxton, U.S. Pat. No. 4,471,785 by Wilson et.al., U.S. Pat. No. 4,817,614 by Hassler et al and in an article "Phase aberration Correction Using Signals from Point Reflectors and Diffuse Scatterers: Basic Principles" Flax et.al., IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, vol. UFFC 37, No. 5, September 1990, pp. 758-767.
Another prior art method for at least partially correcting for focus errors produced by inhomogeneities in the media includes arbitrarily time shifting signals from each element or small group of elements and testing to see if the effect was to increase or decrease the quality of the detected summed signal corresponding to the region in the object space in which focus is to be improved. Each element or small group of elements is corrected in turn until the process converges. Quality of detected summed signal in the region of interest (ROI) may be determined by measuring the "speckle" amplitude, i.e. the mean of the Rayleigh-distributed magnitude, within the region of interest. Examples of such methods are shown in the following articles: "Phase Aberration Correction in Medical Ultrasound Using Speckle Brightness as a Quality Factor," Nock et al, J.Acoust. Soc. Am. 85(5), May 1989, pp. 1819-1833 and "Experimental Results With a Real-Time Ultrasonic Imaging System for Viewing Through Distorting Media," Trahey et al, IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-37, No. 5, September 1990, pp. 418-427.
A typical ultrasonic imaging apparatus may include 64 or 128 transducer elements and associated adjustable signal delay means. Where delays are adjusted individually, or in small groups, when correcting for focus defects produced by velocity inhomogeneity of the object under examination, as in the prior art, a relatively large amount of time is required to make the corrections. Whenever the region of interest changes, or the transducer array is moved relative to the object, the time-consuming correction process must be repeated.